P0171

DTC P0171 Lean Exhaust:

Heated Oxygen Sensor (HO2S) Circuit:

Circuit Description
The VCM applies a bias voltage of approximately 450 millivolts (350-550 mV is normal) between terminals "BL23" and "BL24". The Heated Oxygen Sensor (HO2S) varies the voltage within a range of about 1 volt if the exhaust is rich to .100 volts if lean. DTC P0171 will set if HO2S voltage remains low too long.

DTC P0171 Will Set When
DTC P0171 will set when HO2S voltage is less than .104 volts for more than 70 seconds.

Action Taken (VCM will default to)
VCM will default to "Open Loop."

DTC P0171 Will Clear When
Conditions for fault are no longer present and Tech 1 clear DTCs function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and Fuse #9 (ECM B) fuse is removed for 10 seconds.

DTC Chart Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Oxygen sensor voltage below .35 volt indicates that the fault is still present.

2. By disconnecting the HO2S, the scan tool should display the bias voltage from the VCM.

Diagnostic Aids
Using the Tech 1 scan tool, observe the long term fuel trim values at different RPM and air flow conditions. The Tech 1 scan tool also displays the block cells, so the long term fuel trim values can be checked in each of the cells to determine when the DTC P0171 may have been set. If the conditions for DTC P0171 exist, the long term fuel trim values will be around 150.

^ HO2S wire - An oxygen supply inside the sensor utilizes a reference oxygen which is supplied through the HO2S wires. All HO2S wires and connections should be inspected for breaks or contamination that could prevent reference oxygen from reaching the HO2S.
^ heck for intermittent ground in wire between connector and sensor.
^ Fuel contamination - Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set a DTC P0171.
^ Fuel pressure - System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm. Refer to "Fuel System Diagnosis," CHART A-6. Related Tests, Information and Procedures
^ Exhaust leaks - If there is an exhaust leak, the engine can cause outside air to be pulled into the exhaust and past the sensor. Vacuum or crankcase leaks can cause a lean condition.
^ CKT 413 - If CKT 413 is open, the voltage at terminal "BL24" will be about .45 volt. This may also cause DTC P0139 to set.
^ If all checks OK, the Heated Oxygen Sensor (HO2S) is faulty.